This invention relates to the discharge of particulate material, grain for example, from a nozzle and, more specifically, to a method and apparatus for reducing the "dust" emissions that normally occur when a mass of mixed fines and heavier particles is discharged as a falling stream.
"Dust" arising from lighter or fine particles separating from a particulate product stream falling in open air is a significant problem in a number of industries which must handle particulate materials, such as grain, concrete, sand and various minerals. Such "dust" is usually fine particles of the material itself, rather than dirt. It is objectionable because it results in airborne particulate pollution detrimental to the quality of the atmosphere and to the health of operating personnel in the vicinity of the emissions. Such emissions also result in loss of valuable product and, in some cases, may present a serious danger of explosion.
In the grain handling industry, for example, grain is continually moved into and out of barges, elevators, silos, grain bins, trucks, tank cars and the like which may be located either inside or outside of physical structures. The grain typically is moved through tubes or pipes by auger, gravity, drag conveyors, or belts. Grain dust, which consists of small grain particles, becomes dispersed into the atmosphere when the moving grain stream is discharged from the conveyor. The loss of grain dust into the air is an economic problem because it reduces the amount of grain to be sold. Dust emissions may also be in violation of government regulations controlling the quantity of airborne particulate matter released in the air, and such regulations are becoming increasingly stringent. In addition, grain dust is highly flammable. The dust was often allowed simply to float into the atmosphere, and was lost. Because grain dust is valuable, many grain handling operations now recirculate it in the system. However, such recirculation increases the concentration of the finer and drier particles in the air, thereby increasing the risk of dust explosion. Other systems for controlling grain dust have employed fabric filters to collect and recover the dust. Such filters capture the lighter and potentially most explosive particles. As a result, a smaller amount of dust is required for explosion than if the grain dust were vented to the atmosphere. Another prior dust control method includes the use of a dust separation device which extracts the dust from a moving grain stream by means of a fan, concentrates the dust in a cyclone, conveys it to a holding container and then pumps the dust back into the grain below the surface. Such systems are objectionable because they are expensive and because the reintroduction of the grain dust results in concentrated "pockets" or regions of the dust in the bulk pile. Thus, when the grain is randomly sampled, a sample having a high proportion of dust may be obtained, rather than a more representative sample. This lowers the overall value of the grain. It is also common not to reintroduce the grain dust into the grain at all but rather to treat it in another way, for example, by pelletizing it and selling it as inexpensive cattle feed, or merely by hauling it away to a dump.